Ring Size Converter
Convert ring sizes between US, UK, EU and JP systems.
Overview
The Ring Size Converter maps a ring size between the four most-shopped systems: US numeric (4, 5, 6, ...), UK alphabetical (H, I, J, ...), European numeric (47, 49, 51, ...) and Japanese numeric (8, 9, 10, ...). Each system is anchored to the internal circumference of the band in millimetres, so the conversion is consistent and reversible.
The tool is for online jewellery shoppers, engagement-ring buyers ordering from a foreign retailer and jewellers building a size guide for an international storefront. Long-tail queries it answers include "UK ring size N in US", "EU ring size 54 to US", "Japanese ring size 13 conversion" and "convert ring size mm to US".
How it works
Every system is based on the same underlying measurement: the inner circumference of the band in millimetres. US size 0 is 38.0 mm and each whole-size step adds about 2.55 mm (one ISO 8653 step). UK sizes step by 1.25 mm per half-letter. EU sizes are numbered by the circumference in millimetres minus 40. Japanese sizes are numbered roughly by the inner circumference minus 38.
The converter takes any of these sizes, resolves it to a circumference in millimetres and emits the closest size in every other system. Half-sizes are supported in US and UK; EU is reported to one decimal place when needed.
Examples
US 6 → UK L 1/2 → EU 51.5 → JP 12
US 7 → UK N → EU 54.5 → JP 14
US 9 → UK R 1/2 → EU 59 → JP 18
51 mm circumference → US 5.75 / UK L / EU 51 / JP 11
FAQ
Why is the UK system in letters?
UK ring sizing pre-dates the metric era and was set by alphabet-step gauges in the jewellery trade. Each whole-letter step is about 1.25 mm of circumference; half-sizes (M 1/2, N 1/2) fill in between.
How do I measure my ring size at home?
Wrap a thin strip of paper around the base of your finger, mark where it meets, and measure the length in mm. That's your inner circumference. Or, measure the inner diameter of a ring that fits well and multiply by π.
Does finger size change during the day?
Yes. Fingers swell in warm weather and after exercise. Measure at the end of the day when fingers are at their largest to avoid a too-tight ring.
Should I round up or down between sizes?
Round up. A slightly loose ring can be sized down; a too-tight ring is uncomfortable and risks getting stuck.
Are wide bands sized the same as thin bands?
Wide bands feel tighter because more of the finger is enclosed. Jewellers usually recommend going up half a size for a band wider than about 8 mm.